The current emphasis on the improvement of the economics and the competitive position of commercial nuclear power presents a strong incentive for efforts to reduce reactor fuel costs. A principal means for accomplishing such a reduction is through an increase in the design power output and fission burnup of the nuclear fuel.
Nuclear fuel rods for water reactors generally include a tube or container within which is nuclear fuel in the form of fuel pellets which may have various shapes such as spheres, small particles or cylinders and which comprise fissionable and/or fertile material. Various nuclear fuels may be used, including uranium compounds, thorium compounds, plutonium compounds, and mixtures thereof. Nuclear fuel pellets currently being used in water reactors in the United States are typically circular cylinders comprised of sintered uranium dioxide (UO.sub.2) which has been enriched in Uranium.sup.235 (U.sup.235) up to approximately 5 weight percent (w/o). Outside of the United States, the nuclear fuel in the nuclear fuel pellets can be a mixed oxide in which, for example, plutonium (which is obtained from the reprocessing of spent nuclear fuel) is combined with natural or depleted uranium to form a mixed oxide of plutonium and uranium [(Pu-U)O.sub.2 ] containing up to about 5% fissionable plutonium and which can be used in power reactors otherwise using slightly enriched uranium.
During the course of reactor operations, the nuclear fuel pellet and more particularly an outer layer called the rim which is formed as a result of high values of burnup reached in the pellet periphery, undergoes significant structural change. At high burnup, the rim becomes highly porous and the grains of the fuel fracture into subgrains of 0.5 to 1.0 microns in size compared to a grain structure when initially manufactured of typically 10 microns or larger. The rim structure is mechanically weak, prone to gas release, and has low thermal conductivity due to the high porosity. Although the thickness of the rim depends upon how the rim is defined as well as the local burnup in the rim which in turn depends on the initial enrichment of the fuel pellet, at burnups of 50 GWd/tHM (gigawatt-days per metric ton of heavy metal: thorium, uranium, or plutonium), the thickness of the rim is typically approximately 50 microns (2 mils) and at 60 GWd/tHM the thickness may reach up to 100 microns (4 mils). At an average pellet burnup of 70 GWd/tHM, the thickness may be 200 microns (8 mils).
The formation of the rim structure can significantly impair the ability to achieve high fuel burnup. The rim becomes highly porous, mechanically very weak which increases the possibility of fracture, and concomitantly increases the probability of athermal fission product gas release and decreases the local thermal conductivity which leads to an increase in fuel rod centerline temperature.
The development or formation of the characteristic rim structure is the result of very high burnup values reached in the pellet rim. More particularly, during operation of reactors loaded with uranium dioxide fuel enriched in U.sup.235, neutrons and particularly epithermal neutrons which are produced from fissioning of U.sup.235 in fuel pellets, penetrate a small distance into a thin outer layer of the fuel pellet where they are available for capture by Uranium.sup.238 (U.sup.238) due to resonance peaks to form Plutonium.sup.239 and other plutonium isotopes. The high burnup in the pellet rim is the result of the fissioning of U.sup.235 plus the additional fissioning of fissile plutonium, which as stated above is bred from U.sup.238 in the rim mainly by the capture of epithermal neutrons.
Since the formation of the mechanically weak porous rim structure can significantly impair the ability to achieve high burnup (e.g. above 55-60 GWd/tU pellet average in uranium fuel, where 1 GWd/tU=1 giga-watt day per metric ton of uranium=1 GWd/tHM) and increased power output, it would be an advantage to have a fuel pellet in which the formation of the rim structure is prevented, or at least reduced or delayed.